In automotive electronics, for certain applications, simple and high-precision measurement of a change of conductor resistances is necessitated. This is accomplished by so-called air-mass sensors, for example. The measured quantity of an air-mass sensor, which is often indicated in kg/h of air or gas, may, for example, be used for controlling an injected amount of fuel. The sensors employed in motor vehicles operate according to a thermal principle. The power output of a heated sensor element serves a measured quantity for an air or gas mass current.
Air-mass sensors may particularly be used for the determination of intake air masses of internal combustion engines. For combustion to be complete and, as far as the exhaust gas composition is concerned, optimum, they are indispensable. In conventional air-mass sensors, for example, so-called hot-element anemometers (=thermal air-mass sensor) are employed. The measuring principle in hot-element anemometers consists in detecting the heating-up of a heater element and the cooling-down thereof with respect to the power output (amount of heat released) from the heater element to the air mass flowing past the heater element. Here, for example, are two sensors, one of which detects the temperature of the intaken air and the other one being heated up to a certain excess temperature compared to the ambient temperature, are located in different branches of a Wheatstone bridge circuit. The heated-up sensor serving as a measuring probe is cooled down by the air flow in dependence on the speed and temperature as well as the heat capacity thereof. The additional amount of energy necessitated for maintaining the adjusted excess temperature of the sensor or measuring probe is then a measure for the air mass put through.
In further known variations of air-mass sensors, the temperature change caused by an air flow is measured by at least one pair consisting of a heating and a temperature measuring resistor.
In all cases, the quantity to be measured may be retraced to a change of resistance, the signal of which depends on a temperature coefficient TCR of the material used for the hot-element anemometer. The sensitivity of the measurement is therefore limited to the temperature coefficient TCR.